The invention relates generally to a method for estimating motion contents in video signals, said motion being normally represented by a two-dimensional motion vector field, including horizontal and vertical vector components. The general purpose of such an estimation is to allow high quality temporal transformation or processing of video signals. The invention can be applied in a variety of fields, some important examples being: conversion between different television signal standards, noise reduction of video signals, data compression for digital transmission of television or high definition television signals, and slow-motion devices.
Known methods of generating motion vector fields include recursive pixel gradient methods, phase correlation and block matching techniques. Each technique has its own set of advantages and disadvantages with respect to the following generally desirable features:
A: High tracking range involving the ability to track objects moving at high speed, as a result of using a large search area; PA0 B: High vector resolution involving the ability to generate a detailed description of the motion vectors of moving objects, as a result of discriminating different velocities and directions of movement of neighbouring blocks. PA0 C: High spatial resolution involving the ability to describe different movement of small adjacent areas of moving objects, as a result of discriminating neighbouring pixels with different movement. PA0 D: Robust algorithm involving the ability to distinguish the true motion of an object by generating a motion vector field while suppressing noise and other disturbances; and PA0 E: Cost effectiveness (effective signal processing and simple implementation) involving the ability to use the method commercially for a wide range of applications.
The advantages and disadvantages of the known methods appear from the table below:
______________________________________ Standard Pixel Phase block gradient correlation matching ______________________________________ A: High tracking range NO YES YES B: High vector resolution YES YES YES C: High spatial resolution YES NO NO D: Robust algorithm NO YES NO E: Cost effectiveness YES NO YES ______________________________________
In the standard block matching technique (compare, e.g. U.S. Pat. No. 4,924,310), the video signal is represented by a consecutive sequence of pictures separated in time, and said pictures are divided into blocks of predetermined orientation and size. The block size will determine the spatial resolution of the vector field. To enable comparison or matching of blocks, one has to define a search area with a certain size and shape, a search pattern, according to which the search or comparison will be carried out, and a matching criterion representing the degree of likeness of one block to another. Such a criterion is based on a comparison of light intensity or luminance of the individual picture elements (pixels).
For the purpose of determining the momentary motion vector field, two consecutive pictures are regarded. The blocks, made up of a number of pixels of the current consecutive picture, are processed in order. For every such block a search is carried out in the search area of the previous consecutive picture, according to the search pattern, so as to match said block to a block in the previous consecutive picture. The direction and the value of the relative displacement of the two matching blocks are selected as the motion vector of the block in the current picture. A complete momentary motion vector field is obtained after blockwise processing of the entire content of the current picture.